In WDM communication, an increase of transmission volume, a higher density multiplexing technique with a narrower wavelength band is adopted, wherein laser sources with accurately controlled wavelength have been required. Generally, it is required that a wavelength of an optical signal is stable for long period of time in high density WDM. Therefore, a wavelength monitor is needed for monitoring a change of wavelength of an optical signal and used for controlling to stabilize it. An optical module equipped with a conventional wavelength monitor is disclosed in Japanese Patent Provisional Publication No. 2000-56185 as an example.
FIG. 15 is a diagram showing a configuration of an optical module having a conventional wavelength monitor. As shown in FIG. 15, the conventional optical module comprises:
a laser diode 50 which emits a laser at predetermined oscillation wavelength;
an optical fiber 51 which is optically connected to the laser diode 50 and transmits a laser that emitted from a front portion (In FIG. 15, right-hand side) of the laser diode 50;
an optical filter 52 which allows to pass only the laser beam of a predetermined wavelength;
a beam splitter 53 comprising a half mirror which divides into two directions the laser beam (monitoring light) emitted from a rear portion (In FIG. 15, left-hand side) of the laser diode 50;
a 1st photodiode 54 which receives one of laser beam after divided by the beam splitter 53 and passed the optical filter 52;
a 2nd photodiode 55 which receives another laser beam after divided by the beam splitter 53;
a Peltier module 56 which controls a temperature of the laser diode 50;
and a control unit 57 which controls the Peltier module 56 to control the wavelength of laser diode 50 responding to a PD current generated by the 1st photodiode 54 and the 2nd photodiode 55.
A focusing lens 58 for coupling the laser emitted from the front portion of the laser diode 50 into the optical fiber 51 is placed between the laser diode 50 and the optical fiber 51. In addition, a collimator lens 59, which makes a parallel ray of the laser emitted from the rear portion of the laser diode 50, is placed between the laser diode 50 and the beam splitter 53.
The laser diode 50, the focusing lens 58, and the collimator lens 59 are mounted on the LD career 60. The 1st photodiode 54 and the 2nd photodiode 55 are mounted on the 1st PD career 61 and the 2nd PD career 62 respectively.
The wavelength monitor 63 is composed by the beam splitter 53, the optical filter 52, the 1st photodiode 54, the 2nd photodiode 55, the 1st PD career 61, and the 2nd PD career 62.
The laser diode 50 and the wavelength monitor 63 are placed within the package 64. And a ferrule 65 holding a tip of the optical fiber 51 is fixed to the side of the package 64 by a sleeve 66.
The laser beam emitted from the front portion of the laser diode 50 is focused by the focusing lens 58 and carried incident upon the optical fiber 51 held by the ferrule 65 and transmitted outside.
On the other hand, the laser emitted from the rear portion of the laser diode 50 is made to a parallel light ray by the collimator lens 59 and divided by the beam splitter 53 into two directions, whereof Z-axis direction (transmission direction) and X axis direction perpendicular to the Z-axis direction (reflection direction). The laser beam in the Z-axis direction is filtered in wavelength by the optical filter 52 and sensed by the 1st photodiode 54. The laser beam in the X-axis direction is sensed by the 2nd photodiode 55. The PD current from the 1st photodiode 54 and the 2nd photodiode 55 are fed to the control unit 57, and the control unit 57 controls the Peltier module 56 for controlling a temperature and maintaining a wavelength of the laser diode 50 corresponding to the PD current.
Furthermore, a laser module without such a wavelength monitor has been traditionally used as well, but even for such a conventional laser module, accurate controlling of temperature is needed for a semiconductor laser in order to improve its accuracy of the wavelength control. Refer to Japanese Patent Provisional Publication-No. 1992-75394.
The optical filter 52 furnished in the wavelength monitor 63 described above is made of, for example, Etalon having a temperature dependency in the optical transmission characteristics. One optical filter shows a shifting of 0.01 nm/degree C. for the wavelength dependency of transmission characteristics to the shorter wavelength side. Also, the heat transfers to the optical filter 52 by thermal radiation from the outside of the package 64, therefore the optical filter 52 has a temperature dependency on the case. So the wavelength of the optical signal may not be controlled well accurately in some cases due to a temperature change of the optical filter 52 with the conventional type wavelength monitor 63. Refer to Japanese Patent Provisional Publications No.2002-335036, No.2001-308444.
Furthermore, a laser diode of oscillator has a temperature dependency in the lasing wavelength, for example, a wavelength of a certain semiconductor laser shifts by 0.08 nm/degree C. Utilizing this, a variable wavelength type is made in some cases, however it is needed to control the temperature being constant in order to maintain the wavelength stabilized during its operation.
There are several methods for controlling a wavelength. One method is controlling a temperature by a temperature control unit while monitoring a change of the wavelength by a wavelength monitor, another is controlling a temperature by a temperature control unit while measuring a change of the temperature of the laser diode measured by a temperature sensor such as a thermistor.
As the ambient temperature changes, the amount of heat flow from the package into the laser diode through the bonding wire or the gas changes. In this situation, even if the temperature control for the laser diode and the thermistor is conducted on the same career, in some occasion the control may not follow the rapid change of the surrounding temperature, or when a temperature difference between the laser diode and the thermistor is produced, accurate measurement of the temperature is not taken. Consequently, it may cause a problem that the lasing wavelength cannot be stabilized.
In addition, the laser diode is cooled down to approximately 0 degree C. in order to have a wider variable range of the wavelength, while it may located where ambient temperature reaches 70 degree C. approximately at some conditions, wherein the amount of heat flow from the package becomes larger and hence it is likely to cause a large temperature difference between the laser diode and the thermistor.